The present invention follows from some ideas already in the public domain. U.S. Pat. No. 5,410,913, by Blackburn, describes a “Remote Indicating Liquid Level Sensor,” and FIG. 1 shows a diagram from this patent. The problem solved by Blackburn is a way to have an electronic readout of a liquid level in a tank without putting electrical components into the tank. This was accomplished by placing a permanent magnet on an apparatus inside the tank, and having electronic magnetic detection outside the tank.
Specifically, the internal permanent magnet is on a bent arm that pivots in a circle as the main shaft twists about its central axis. In this case the permanent magnet rotates around a large diameter, in the range of several cm. The motion of the magnet is detected by an external circular array of 9 magnetic sensors. The sensors are arranged with a regular angular spacing about the circle so that only 1 or 2 sensors are detecting the presence of the magnet at any one time. The magnetic sensors described are magnetic-reed switches. These electrical switches open and close when the magnetic force upon a thin strip of ferromagnetic material is sufficient to bend the strip into contact with a second electrical contact. The magnetic force is generated by the strip's interaction with an externally applied magnetic field. When the field is stronger that a certain threshold level, the strip will bend and close the electrical contact. When the externally applied magnetic field is smaller than said threshold level, the strip will not be bent enough to close the electrical contact.
There are solid state magnetic switches, described in the public domain, that open and close electrically with no physical motion needed. Also in the public domain are magnetic sensors that detect the rotation angle of a shaft.
Also in the prior domain, there are magnetic sensors that count the number of revolutions that a shaft makes about its axis.
The first three figures come directly from U.S. Pat. No. 5,410,913. FIG. 1 shows a cross section of an electronic remote liquid level sensor. This sensor system is placed through a hole at top of a tank, and has electronic connections that are outside the tank cavity. The design allows the tank to be sealed off from the external atmosphere, and to be at a different temperature and pressure than the surroundings. These requirements are determined by the specific application. There are two main parts of the system shown here, assembly A and assembly B. Assembly A is the mechanical socket and assembly is an electronic sensor module. These two assemblies are normally mechanically joined together. Here, they are separated to enable easier viewing and explanation of the drawing.
Assembly A is located at the interface between tank wall 13 and sensor base plate 10. Sensor hole 12 is cut into tank wall 13 to accommodate the level sensing assembly. Guide tube 14 provides a structural support for the vertically extended portion of the level sensing assembly. It has two ports 27, 28 that allow liquid to fill tube cavity 38 so that the liquid level inside guide tube 14 is the same as liquid level 39 in the rest of the tank.
The vertical location of float 15 follows liquid level 39 as it goes up and down along the Z axis 16. We set the origin of our coordinate system to be at the intersection of X axis 8 and Z axis 16; Y axis 9 is into the page and not shown in FIG. 1. This origin is on the top surface of guide tube bottom plate 26. When liquid level 39 is below Z=0, float 15 falls to Z=0 but is prevented from falling further by guide tube bottom plate 26. When liquid level 39 is higher than the height of float 15, the precise amount of float 15 that is above and below liquid level 39 depends on the ratio of their respective specific gravities. So long as the specific gravity of float 15 is smaller than that of the liquid, it will ride on the liquid at a vertical position such that liquid level 39 is between its top and bottom as shown in FIG. 1.
Rigid rotating shaft 19 has permanent magnet 22 affixed to it upper end. It is bent at a right angle so that as radially directed arm 21 rotates about the axis of rotation (Z axis) 16, permanent magnet 22 goes in a circle in groove 30 cut into fixture top plate 29. Shaft hole 24 is cut into sensor mating plate 10 so rigid rotating shaft 19 can pass through. Mechanical support for the weight of rigid rotating shaft 19 is provided by raised ring 25 at the upper end, and the guide tube bottom plate 26 at the lower end. The lower end is allowed to rotate freely about axis of rotation 16. However, no motion along this axis is allowed due to the already described mechanical constraints.
A precise mechanical relationship between the vertical position of float 15 and its angular displacement is maintained by the physical interface between guide tube 14, rigid rotating shaft 19, and float 15. There is a slide-able key interface between float 15 and rigid rotating shaft 19 through the use of key lugs 20 on float 15. There is a slide-able groove—rail interface between float 15 and guide tube 14 through use of guide rails 17 on guide tube 14, and grooves 18 on float 15. Rigid rotating shaft 19 has a twist in it so that the direction normal to its face is changing smoothly as a function of distance Z from the bottom. Twisted rigid shaft upper portion 19′ is at a distance Zfull from the bottom, and has a normal rotation angle theta full that is selected by the designer to meet the needs of a specific application. In this diagram, it is shown that upper portion 19′ is twisted a full 360° from bottom portion of rigid rotating shaft 19. It is not a requirement of the present invention that this twist be exactly 360°; any values are included. Arm 21 carrying permanent magnet 22, points to the angle perpendicular to the normal to upper portion 19′. A general linear relationship between rotation angle of permanent magnet 22, (rigidly rotating with upper portion 19′) and liquid level 39 (and therefore the Z location of float 15) can be written.
Use θLevel as a variable to describe rotation angle 102, and ZLevel as a variable to describe the vertical position of liquid level 39; θFull and θEmpty are constants that describe the rotation angle 102 at Full and Empty conditions; ZFull and ZEmpty are constants that describe the vertical position of liquid level 39 at full and empty conditions. Then, the linear relationship between θLevel and ZLevel can be written as:θLevel=θEmpty+(θFull−θEmpty)*(ZLevel−ZEmpty)/(ZFull−ZEmpty)  (1)
Solving for ZLevel:ZLevel=ZEmpty+(ZFull−ZEmpty)*(θLevel−θEmpty)/(θFull−θEmpty)  (2)
Assembly B contains nine magnetic reed switches 41-49 and resistors 61-69, which are mounted on PCB 35. Switches 41-49 are arranged in a circular pattern that is co-axial with the axis of rotation 16, and at a radius such that the magnetic fields from permanent magnet 22 are large enough to trip the switches but only when at or near the same Rotation Angle as a given switch.
Electrical interconnect flexible wires 33, 34, carry electric signals to and from PCB 35. Sensor module top cover 36 and sensor module bottom cover 37 provide protection and mechanical support to PCB 35. Integrally formed snap fingers 31 in Assembly A provide permanent or temporary mechanical engagement to notches 32 in Assembly B.
A top view of sensor base plate 10 and fixture top plate 29 is shown in FIG. 2. Snap fingers 31 are arranged at regular angular intervals about the axis of rotation 16 as are bolt through holes in plate 10. Together, these structural features fix the position of switches 41-49, while permitting permanent magnet 22 to rotate about the axis of rotation 16.
A top view of PCB 35 with its components is shown in FIG. 3. Switches 41-49 are normally open, meaning they do not conduct electrical current under “low magnetic field” conditions; but under high magnetic field conditions they close, which makes their resistance zero. When permanent magnet 22 passes near a specific switch, that switch goes to closed.
Terminals 50-51 are the external electrical points of measurement for the circuit through the use of a simple resistance measurement. Resistors 61-69 are connected electrically in series between terminals 50 and 51. Magnetic reed switches 41-49 each have one end electrically connected to terminal 51 and their other end connected to a contact point between two of resistors 61-69. If none of switches 41-49 are closed, the net measured resistance between terminals 50 and 51, is the sum of all resistors 61-69. If no switches are closed but switch 49, the net measured resistance will be the sum of resistors 61-68. If no switches are closed but switch 48, the net measured resistance will be the sum of resistors 61-67. And this logic works for all switch locations so that as permanent magnet 22 rotates through its path from 0 to 360 degrees, the measured resistance between terminals 50-51 increases in discrete steps as each of switches 41-49 close.
Generally, it is an electronic remote magnetic float-based rotating shaft liquid level sensor system for a tank. The present invention uses the same or similar rigid shaft rotation mechanism, but an improved magnetic sensing system. The next two figures explain the magnetic detection geometry. This is both the position and orientation of magnetic angle sensors and of permanent magnets whose rotation is detected by the angle sensors.
A perspective view and a cross section view of the magnetic angle detection geometry are shown in FIG. 4 showing the positional relationship between the magnetic field angle sensor chip and the permanent magnet. Permanent magnet 105 rotates about the axis of rotation 16 with the rotation direction 101. The magnitude of its rotation is given by rotation angle 102. An electronic magnetoresistive sensor chip is situated on or near the axis of rotation 16. Its internal sensing elements have a designed axis of sensitivity along X axis 8 and Y axis 9. The magnetic field angle sensor chip 103 should be fixed with respect to the detection coordinate axes, and not move when magnet 105 rotates. Magnetic field angle sensor chip 103 is mounted on Printed Circuit Board (PCB) 104. There is a designed separation S 106 between angle sensor chip 103 and the top surface of permanent magnet 105.
Each sensing element in angle sensor chip 103 has 2 output leads for a total of 4 output leads. The voltage between the pair of output leads for the X axis sensor is plotted as curve 110 in FIG. 5. The voltage between the pair of output leads for the Y axis sensor is plotted as curve 111 in FIG. 5. These curves represent the voltage change as a function of the rotation angle 102.
The above described general purpose magnetoresistive rotation sensor is known in the art. For example, Application No. 201110130222.1 and 201110130202.4 are two patents that describe a kind of design for potential applications magnetoresistive sensor element in a magnetic field angle sensor. These patent documents are hereby incorporated into the present application by reference.
However, in the above prior art there are some defects, such as the kind of detection system used to detect the permanent magnet as well as the large diameter guide tube that is needed; also the mechanical magnetic sensors are prone to failure.